"A 'heart song' is really a love song," Emmanuel said. "Love has many facets—love of family, romantic love. So we started there and then looked for melodies that could be played with heart."

"And we tried to choose songs that people have heard," Knowles added. "So that when they hear them, even though we're not singing or they may not know all the words, they have a connection to these songs in their own hearts."

Regarding their playing on the album, Emmanuel said, "It was a wakeup call to not overplay, to be respectful and serve the music properly.”

“When you're being respectful to the melody, you're also respecting the listener's previous experience with that song," said Knowles. "We play into their expectations while at the same time surprising them now and then."

Emmanuel and Knowles will kick off a tour in support of Heart Songs in San Francisco on January 8, 2019.